Unifoliate Leaf Research Articles

Vascular Anatomy of the Seedling of Melilotus alba

1. The primary root of Melilotus alba is characterized by a triarch radial protostele. 2. The first indication of a change from the triarch exarch arrangement of the xylem of the primary root in the young seedling is the development of diarchy or tetrarchy in the lower hypocotyl. 3. If tetrarchy occurs, it is invariably replaced at a higher level in the hypocotyl by diarchy. 4. The exarch ridge of xylem on the intercotyledonary axis gradually disappears at higher levels in the hypocotyl, and has no direct connection with the primary xylem of cotyledonary or plumular traces. 5. The exarch-endarch transition is continued in the formation of the cotyledonary traces and is completed in the main vascular bundles at the base of the cotyledonary blades. 6. A conspicuous pith differentiates in the hypocotyl, the stele enlarges, the cortex becomes more prominent, and a cutinized epidermis is present in this region in the young seedling. An endodermis with Casparian strips is present throughout the greater length of the hypocotyl. 7. A continuous, primary vascular system exists from root, through hypocotyl, into cotyledons. 8. The median vascular bundle which supplies the unifoliate leaf at the second node develops from meristematic tissue in a seedling several days old. The first indication of the differentiation of this collateral endarch bundle is the appearance of a single xylem element in the upper hypocotyl on the intercotyledonary axis, above the disappearing protoxylem of the third ray or on the opposite side. 9. In seedlings seven to ten days old, the median trace to the first trifoliate leaf at the third node, as well as the four bundles which continue upward to form the two lateral traces to the unifoliate leaf and the first trifoliate leaf, respectively, are well differentiated as endarch collateral bundles in the upper hypocotyl. These six primary bundles arise from meristematic tissue and appear to have no direct connection with the primary exarch strands in the lower hypocotyl. 10. Buds arise early in the axils of the cotyledons, the unifoliate leaf, and the first trifoliate leaf. The cotyledonary buds do not ordinarily develop into branches the first season. 11. Cambial activity occurs early in the primary root; fibers are prominent in the secondary xylem and phloem. As a result of cork cambium, which arises in the pericycle, a prominent cork is formed and the cortex of the root disappears. 12. The square stem is characterized by a dictyostele, with collateral endarch bundles. 13. The similarity between the method of transition from root to stem structure in Melilotus alba and the types described in M. arvensis, Medicago, Trigonella, Ononis, and Trifolium indicates a close structural and phylogenetic relationship between these species in the Trifolieae.